This can all be done by overriding the template, simply copy the file called templates/loop/sold-out-flash.php and templates/single-product/sold-out-flash.php to your theme's folder under WooCommerce folder keeping the directory structure as it. So it should be yourthemefolder/woocommerce/loop/sold-out-flash.php and yourthemefolder/woocommerce/single-product/sold-out-flash.php and make the changes in those files.